feat: Add GitHub Copilot instructions for repository

[Copilot]

Configures repository-specific guidance for GitHub Copilot via .github/copilot-instructions.md.

Changes

• Created .github/copilot-instructions.md with 123 lines covering:
  • Project structure: Rust workspace with gix (library) and gix-* plumbing crates
  • Development practices: test-first, no .unwrap(), thiserror for libraries, purposeful conventional commits
  • Build/test: just test, cargo build --release --no-default-features --features {lean,small}
  • Architecture: plumbing (low-level, references) vs porcelain (high-level, convenient)
  • Crate stability tiers and release process via cargo smart-release

Context

Provides AI coding assistants with essential context about gitoxide's conventions:

• Error handling (comprehensive chains, explicit .expect() with reasoning)
• Commit style (changelog-driven with ! suffix for breaking changes)
• Feature flags (small, lean, max-pure) affecting build configuration
• SHA256 preparation via gix_hash::ObjectId
• Plumbing/porcelain distinction affecting API design patterns

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• gh.io
  • Triggering command: curl -s REDACTED (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

---

This section details on the original issue you should resolve

<issue_title>✨ Set up Copilot instructions</issue_title>
<issue_description>Configure instructions for this repository as documented in Best practices for Copilot coding agent in your repository.

</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes ✨ Set up Copilot instructions #2243

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

[EliahKagan]

Isn't this either doc:/docs: or not requiring of a conventional commit?

Even if so, I don't think it's necessary to change it--especially since this change won't be reflected in any changelogs because it doesn't modify a file that is seen to be inside any crate. I mention this mainly in case my understanding of the style guidlines are mistaken. If/when I make changes intended to affect agents, should I use conventional prefixes?

[Byron]

Even though I simply forgot about this, it's true that such a message won't make it into any changelog.
Please feel free to use this convention though, maybe it shows up, and if not, there is no harm either.